1. Field of the Invention
This invention relates to offset lithographic printing presses having a spray rail and series of spray nozzles for supplying dampening fluid to the press during printing operations. More particularly, this invention relates to an improved spray rail that creates a more even spray pattern over the dampener of a printing press for improved quality of prints, by introducing air into the dampener solution prior to exit through the spray nozzles.
2. Description of Prior Art
Lithography is the art or process of printing from a flat stone or metal plate. Lithography is distinguished from other forms of printing, in part, by the absence of relief to distinguish between the areas to be printed and those to be left blank. The lithographic method is based upon the repulsion between immiscible liquids, such as oil-based inks, and water. The process begins by the etching of a design on a surface. A grease-like material that is attracted to the etched area is then applied to the surface. The greased areas of the surface repel water ensuring that only ungreased areas of the surface become saturated with water. The printing ink subsequently applied will be repelled by the ungreased areas that are saturated with water and absorbed by the greased areas. This creates the design in ink on the surface.
To ensure that printing ink is not absorbed in areas that are ungreased, and thereby improve the quality of the print, a continuous supply of water must be uniformly applied to the press during the printing operation. If water is not evenly applied, stripes occur in the blank portions of the printed article. This is known as striping and can be experienced to varying degrees depending upon the uniformity of the water coverage. The more uniform the application of water, therefore, the better the quality of the resulting print.
To reduce the variations in the film of water applied to the printing plate, water is not applied directly to the plate. Instead, water is first applied to a dampener or roller and subsequently transferred to the printing plate. Although some of the irregularities in coverage can be removed by indirect application of water to the printing plate, the uniformity in water coverage on the printing plate and consequent quality of print can be further enhanced by achieving a uniform water film on the dampener or roller itself.
Various means of applying water to the dampener have been utilized in attempts to improve the uniformity of the water covering on the dampener. These include partially immersing the dampener in a fountain and the utilization of the action of flicker brushes to coat the dampener with water. Many of these however have proved to be erratic in their achievement of uniformity.
The use of a spray rail comprising a series of spray nozzles to spray fluid onto a dampener has proved quite successful. The spray pattern achieved, although fairly uniform, does result in some variation in the fluid coverage achieved. Over long periods of time, the spray nozzles are prone to clogging. The blockages created may merely reduce the uniformity in spray coverage and therefore the quality of a print. Alternatively, the blockages may completely block the exit of fluid from the spray nozzle creating large areas of dark striping on the final print. When the nozzles are completely blocked, the press must be stopped for cleaning of the nozzles. The frequent stoppages necessary to clean the nozzles reduces operating efficiency. It is desirable therefore to be able to consistently apply a uniform film of water to the printing plate so that prints of higher quality can be consistently achieved. For this reason it is also desirable that nozzle clogging be reduced.
The printing press dampener of this invention applies a water film of improved uniformity to the printing press by improving the uniformity of the spray emitted from the spray rail and reducing the likelihood of blockage. This is achieved by introducing air into the water spray prior to exit through the spray nozzles.